Semiconductor materials formed from group-III-group-V compounds are primarily used in light-emitting diodes (LEDs) based—among others—on their property to emit light in a broad wavelength range. In the following “III” and “V” denote elements of the corresponding main group of the periodic table of chemical elements. In particular, group-III-nitride compounds turned out to be specifically suitable for the green to ultraviolet wavelength range, for example GaN (gallium nitride), InGaN (indium gallium nitride), AlGaN (aluminium gallium nitride) and AlGaInN (aluminium gallium indium nitride).
The above referred to compounds are commonly grown by metal organic vapour phase epitaxy (MOVPE), hydride vapour phase epitaxy (HVPE) or molecular beam epitaxy (MBE), etc. on a start substrate, in order to manufacture a monocrystal. Unfortunately, GaN-monocrystals substrates having a satisfactory size on which to grow the compounds are generally not presently available. Thus, “foreign” substrates, wherein materials comprising an almost compatible crystal system are utilized. For gallium nitride, sapphire (Al2O3) or silicon carbide (SiC) are typically used. When forming the final end products, the substrates are removed from the gallium nitride-monocrystal via chemical or mechanical methods, etc.
Gallium nitride (by which term in the following also shall be comprised its ternary or quaternary compounds) includes a hexagonal wurtzite crystal grid and thus has a similar crystal structure as sapphire, which comprises a trigonal corundum-grid. Crystal growth is currently, generally performed along the c-axis [0001] during epitaxy, i.e., perpendicular to the c-plane (0001), the c-axis being the symmetry axis of the hexagonal structure. The surface of the sapphire substrate (wafer) provided as a substrate surface on which the crystal is grown is chosen to have lattice constants sufficiently close to those of the crystal c-plane to support acceptable growth of the crystal. The c-plane (0001) thus also characterizes the final GaN-crystal as it is then utilized, for example, in LEDs.
Due to the polar structure of the crystal along the c-axis [0001], considerable piezoelectric fields may arise in the case of GaN in conjunction with the crystal geometry. A band shift may be generated along the c-axis from which the so-called quantum confined stark effect (QCSE) may result. In the QCSE, distances between wave functions of electrons and holes in quantum wells (QW) produced along this direction by alternating layer sequences, are increased. The increase leads to a red shift and reduced recombination rate of the electrons and holes in the LEDs, etc.
The c-plane as the surface of the GaN-monocrystal is thus denoted as “polar”. In contrast thereto, the m-planes {1-100} or a-planes {11-20} in the GaN-monocrystal, which are perpendicular to the c-plane, are nonpolar, since the Ga and N-atoms are positioned in the same plane. Consequently, many attempts were made to manufacture GaN-monocrystals having surfaces, which are correspondingly oriented at the m- or a-planes.
Since, however, as described above, foreign substrates have to be used as a start material, the problem arises just in these cases, that stacking-faults, etc. may occur.
Nevertheless, Okado N. et al.: in “Direct growth of m-plane GaN with epitaxial lateral overgrowth from c-plane sidewall of a-plane sapphire”, Appl. Phys. Expr. 1 (2008), page 111101 report that nonpolar GaN (m-plane), or {1-100}, may be grown via MOVPE on a sapphire substrate having {11-20}-orientation (a-plane).
For this purpose, trenches separated by ridges are formed in the sapphire substrate by means of reactive ion-etching (RIE). Lateral facets of the ridges include an inclination of 68 or 79 degrees, respectively, with respect to the horizontal direction. The top face of the ridges is masked by a SiO2 layer having a thickness of 200 nm, in order to inhibit growth of GaN on top of the same. The top faces of the ridges have a width of 4 μm, while the trenches have a corresponding width of 2 μm. The bottom face of the trenches has a width of only 0.5 μm due to the inclined facets.
The epitaxial growth process started horizontally in the trenches in c-direction [0001] at the facet, which is more inclined (79 degrees), since the angle between the c-direction and the surface normal herein amounts to “only” 11 degrees. After reaching the opposite wall, the growth continues vertically with lateral overgrowth of the ridges, wherein the direction in the gallium nitride now corresponds to an {10-10}-orientation (m-plane) due to a 30-degrees-rotation. In view of the lateral overgrowth over the ridges, the individual layers finally merge upon further growth.
A surface roughness measurement conducted with an atomic force microscope (AFM) amounted to 1.1 nm (are size 5×5 μm, RMS), whereas the crystal quality has been determined by an x-ray measurement via the Rocking Curve to yield a full width half maximum (FWHM) of 500 arc seconds azimuthally in the <10-10>-direction, or of 650 arc seconds azimuthally in the <0001>-direction for the (10-10)-surface of the grid.
The formation of a semipolar GaN-layer was thereafter described by Okada, N. et al.: in “Growth of semipolar (11-22) GaN-layer by controlling anisotropic growth rates in r-plane patterned sapphire substrate”, Appl. Phys. Expr. 2 (2009), page 091001. “Semipolar” denotes, polarity values between the extremes of the c-plane polarity and the m- or a-plane polarity.
By an appropriate choice of the process conditions in the MOVPE-process it was possible to omit the masks upon the ridges. The top face of the ridges were oriented along the r-plane (1-102) of the sapphire substrate, whereas the lateral facets were again inclined (about 32 degrees with respect to the horizontal direction), however, this time its normals were oriented parallel to the c-plane inclined within the sapphire substrate, such that a single-sided growth in c-direction [0001] in the trench was guaranteed. The r-plane is not perpendicular to the c-plane. Due to the 30 degree-rotation about the c-axis upon growth on the sapphire, the gallium nitride crystal grid achieves by its [11-22]-direction in vertical direction the same spatial orientation as compared with the [1-102]-direction of the crystal grid of the sapphire substrate.
Regarding crystal quality, rocking curve-measurements yielded full width half maximum (FWHM) values of 720, or 319 arc seconds, parallel and perpendicular to the c-direction for the (11-22)-grid planes, respectively.
The growth has been supported particularly by the almost similar angles between the r-plane and the c-plane of the sapphire and between the (11-22)-plane and the c-plane of the gallium nitride.
From document U.S. Pat. No. 7,220,324 B2 it is known to grow a GaN-layer having {10-11}-orientation on a {10-10}-spinel substrate (MgAl2O4). The spinel substrate thereby is not structured, since the method of forming ridges and trenches followed by a two-step growth due to the facet inclinations and the small area sizes, is considered insufficient and complex.
From document U.S. Pat. No. 7,645,688 B2 it is known to use a (11-23) oriented sapphire substrate to grow thereupon a nonpolar gallium nitride layer oriented in the direction of the m-plane (<10-10>-directions). The substrate is substantially unstructured. One would basically select an r-plane sapphire substrate in order to grow nonpolar (11-20)-GaN, i.e., a-plane GaN. However, it is proposed therein to use (11-23)-oriented sapphire substrate instead, since the grid dimensions in both planes (sapphire-GaN) are compatible with each other and m-plane-GaN is considered being more stable than a-plane-GaN.